Numerous scientific and technical applications require laser beams of adjustable, yet precisely defined wavelength. This necessitates the laser device to emit the radiation of a single, precisely defined mode of the laser resonator.
By means of appropriate geometry of the optical resonator, it is possible to ensure that only one transverse mode of the system oscillates. Beyond that, however, it is also necessary to restrict it to one longitudinal mode in order to afford the desired limited spectral bandwidth of the laser beam. The so-called single longitudinal mode (SLM) operation must be ensured by means of frequency-selective optical structural elements, the optical band-pass filters. The high spectral resolution of a band-pass filter requires an active regulation of the filter parameters in order to ensure that only one longitudinal mode oscillates in the laser resonator. A detailed illustration of the design of such band-pass filters can be found in Koechner's 1992 volume on Solid State Laser Engineering 
In the past, the adjustment of band-pass filters was accomplished by the use of a so-called “hill-climbing” servomechanism, utilizing an analog modulation of the mid-setting, followed by analog demodulation of the intensity signal. Modulation took place at a fixed frequency (for example six kilohertz). The U.S. Pat. No. 3,543,181 by Lee at al. (1968) describes the construction of a simple embodiment of such a control.
With discrete analog circuits and/or processes of the aforesaid type it was heretofore technically difficult and/or economically unwarranted to normalize the control input signals. Hence, use was made either of simplified calculations, for example subtraction in lieu of division (that is to say, the first element of Taylor's series expansion), or by varying the gain of the next following control elements as a function of the laser power. Inasmuch as this involved, in part, a rough simplification of the required mathematical operations, the results were correspondingly unsatisfactory. With a variation of the laser output, the adjustments became unstable and the single longitudinal mode of operation was not ensured, leading to a so-called mode-hopping.
The analog modulation oscillators in use are limited to a fixed frequency. Since the modulation frequency is modulated upon the laser beam, it is necessary for the user to take this into account in using the laser, being correspondingly limited for example in the lock-in frequency of his own structures.
Most band-pass filters consist of several stages, all of which must as a rule be adjusted, which always involves the risk that the individual control circuits may interfere with each other. An analog control affords no chance to recognize this and afford suitable remedial measures. Optionally, individual components are not adjusted, which again unnecessarily limits the spectral tuning range of the laser system.
Customarily, the adjustment parameters of analog control circuits are established with the aid of discrete structural elements, such as for example trimming potentiometers. Such elements can only be adjusted manually and in sequence, a time-consuming procedure not reproducible beyond certain limits.
The “interim results” of an analog computation are not directly available, but must be laboriously prepared for the user with the aid of additional metering devices, such as for example the oscilloscope.
The operating amplifiers utilized in analog circuits require compensation of the so-called offset voltage, whereby the multi-stage control circuits utilized at this time involve the addition of offset voltages, that is to say, the adjustment is subject to distinct temperature and time-dependent drifts.